1. Field of the Invention
The present invention relates to a noise filter and, more particularly, to a noise filter employing a 4-terminal capacitor for filtering noise signals having a high frequency, such as from several 100 KHz to several 100 MHz.
2. Description of the Prior Art
Lead lines for transmitting signals often pick up high frequency noise signals. For example, when the lead line is disposed near a source of a high frequency signal, it picks up the high frequency noise electro-magnetically. Also, when an electric circuit employs, example, a switching regulator that alternately makes and breaks a current path as a power source, electromagnet interference noise appears on a power supply line to the electric circuit, thereby causing various troubles in the electric circuit. To eliminate such high frequency noise, a noise filter defined by a 2-terminal capacitor, such as shown in FIG. 1, is generally connected between a power supply line A and a ground line B.
The 2-terminal capacitor of FIG. 1 constitutes a ceramic dielectric plate 1, a pair of electrodes 2 and 3 deposited on opposite surfaces of the plate 1, respectively, a pair of terminal legs 4 and 5 extending from the electrodes 2 and 3, respectively, and a synthetic resin coating 6 covering the capacitor for insulation purposes. An equivalent circuit of the 2-terminal capacitor of FIG. 1, when it is connected between the power line A and ground line B, is shown in FIG. 2, in which a capacitor C1 represents capacitance between the electrodes 2 and 3, while inductors L4 and L5 represent inductances of the terminal legs 4 and 5, respectively. When the signals having various frequencies are transmitted through the power line A, a signal having a certain high frequency is filtered to ground through an L-C filter circuit including the capacitor C1 and inductors L4 and L5, while the remaining signals are transmitted through the power line A without any disturbances. The frequency of the signal filtered through the L-C circuit is determined by both the capacitance and inductance between the lines A and B. Accordingly, the inductance of the terminal legs 4 and 5 greatly influences the determination of the signal that can be filtered through the L-C circuit. From the view point of practice, the inductance in the L-C circuit should preferably be as low as possible. However, in the 2-terminal capacitor of FIG. 1, the inductors L4 and L5 have such a high inductance that only signals having a considerably low frequency as compared with that of the high frequency noise are filtered. Therefore, the high frequency noise signals can not be effectively filtered by the 2-terminal capacitor of FIG. 1.
To improve the filtering effect, a noise filter defined by a 4-terminal capacitor shown in FIG. 3 has been proposed. The 4-terminal capacitor of FIG. 3 has a ribbon-shaped terminal member 7 soldered to the electrode 2 and another ribbon-shaped terminal 8 soldered to the electrode 3. When in use, the ribbon-shaped terminal member 7 is connected in series with the power lead line A, while the other ribbon-shaped terminal member 8 is connected in series with the ground lead line B. An equivalent circuit of the 4-terminal capacitor of FIG. 3 is shown in FIG. 4 in which inductors L7' and L7" represent inductances in the wing portions of the ribbon-shaped terminal member 7, and inductors L8' and L8" represent inductances in the wing portions of the ribbon-shaped terminal member 8. As understood from the equivalent circuit of FIG. 4, no inductance, but only a capacitance 10 is present between the lines A and B.
Although the 4-terminal capacitor of FIG. 3 filters high frequency noise signals, the filtering effect is not sufficient particularly when noise signals have a high frequency, such as from several 100 KHz to several 100 MHz, as caused, for example , by the employment of a switching regulator.